1. Field of the Invention
The present invention relates to a thermistor as an electronic part.
2. Description of the Related Art
A negative temperature coefficient thermistor (hereinunder referred to as "NTC") in which the resistance lowers with the rise of a temperature, a positive temperature coefficient thermistor (hereinunder referred to as "PTC") in which the resistance increases with the rise of a temperature, and a critical temperature thermistor are known. The temperature of the apparatus on which such a thermistor is mounted is detected by measuring the electrical resistance of the thermistor.
The NTC, which is generally composed of Mn, Co, Ni and Cu, is easily affected by an oxidizing atmosphere, while the PTC, which is generally composed of BaTiO.sub.3, is easily affected by a reducing atmosphere. Therefore, these thermistors are coated with glass except for the portions at which electrodes are formed, before they are mounted on a substrate. FIG. 1 shows such a thermistor mounted on a substrate. As shown in FIG. 1, the surface of a thermistor element 11 is coated with glass. Both side surfaces of the thermistor element 11 are not coated with glass because electrodes are formed thereon. A silver paste is pasted to the side surfaces of the thermistor element 11 which are not coated with glass so as to form electrodes 13. Connecting portions 17 for receiving bonding wires 15 are formed on the upper surface of the thermistor element 11. A thermistor 19 as an electronic part produced in this manner is mounted on a substrate 21 and bonded with the bonding wires 15.
This thermistor 19 is defective in that the resistance changes depending upon the size and the thickness of the electrodes 13. It is therefore necessary to paste the silver paste to a uniform thickness on thermistor elements in order to keep the resistance uniform in the thermistors produced. However, it is difficult to paste a silver paste to a uniform thickness because the thickness of the applied paste largely varies in accordance with the viscosity and the quantity of the silver paste. As a result, it is difficult to produce the thermistors 19 having a uniform resistance, so that the yield of the thermistors 19 is conventionally low.
In addition, since the glass coat and the electrodes 13 are directly provided on the thermistor element 11, the electrodes 13 are sometimes peeled off when the bonding wires 15 are soldered, which changes the resistance of the thermistor 19, or the thermistor 19 is sometimes broken by thermal shock at the time of soldering.